Typically, propeller-driven aircraft incorporate noise emission controls in two main component areas: (1) the propeller and (2) the exhaust system.
It is known in the art that effective reduction of the noise burden imposed on third parties is achieved only by simultaneously reducing the speed of rotation of the propeller and by muffling the noise of the exhaust. This is particularly so in the case of high-performance engines that have high power densities at high rotational speeds.
It is known that, to minimize the noise generated by propellers in high-performance engines, reduction gearing is needed to reduce the high rotational speed of the crankshaft and to increase the torque on the propeller shaft. In principal, a propeller should only rotate at such a speed that its peripheral velocity is lower than the speed of sound. If the speed of the blades of a propeller approach or exceed the speed of sound, such high speeds result in unacceptable noise emissions and losses of operational efficiency.
The diameter of the propeller factors into the maximum speed of rotation before the propeller begins to generate unacceptable levels of noise. Smaller diameter propellers permit higher speeds of rotation because of the peripheral velocity of smaller propellers is less than that of larger diameter propellers. However, smaller diameter propellers are less efficient than larger diameter propellers. Accordingly, the propeller designer's challenge is to select the best possible compromise between efficiency (i.e., propeller diameter) and the speed of rotation of the propeller for a given power output. For power outputs in the range from 50 to 300 kW, such optimal propeller diameters are typically in the range from 1.5 to 2.2 m. The associated speed of rotation of the propeller is calculated from the selected blade-tip velocity which, as discussed above, should be lower than the speed of sound, and generally be in the range from 200 to 270 m/sec.
Other factors that impact on propeller performance and noise emission are the moment of inertia of the propeller and the moment of inertia of the drive system to which the propeller is connected. The moments of inertia of the propeller and drive system can create operational imbalances that contribute to noise generation during operation of the aircraft engine.
The diameter of the propeller contributes to the moment of inertia of the propeller. The larger the diameter of the propeller, the greater the moment of inertia. Following this rule, a relatively large propeller diameter means that the propeller has a relatively high moment of inertia.
Each of the other components of the engine that rotate together with the propeller contribute to the moment of inertia of the propeller system. The propeller system components include, for example, the components of the drive line such as the crankshaft and the reduction gearing that is arranged between the crankshaft and the propeller.
As a general rule, the moments of inertia of the propeller and propeller system constitute an oscillating system that is characterized by a plurality of natural frequencies (fundamental component and harmonics). The operation of this oscillating system is excited by the torque pulsations of the reciprocating engine. If the frequency of excitation coincides with a natural frequency, this results in resonant vibrations that can produce unacceptable vibration of the drive unit. This results in noise.
Moreover, vibrations created by the oscillating system can also result in excessive wear of the engine components. If present, a high level of local acceleration can produce vibrations that may damage the engine or its associated components. In addition, the creation of such vibrations can lead, for example, to loss of auxiliary assemblies (e.g., generators) or to the failure of various engine components (which is a phenomenon referred to as “vibration failure”).
Furthermore, resonant vibrations in the intermediate gearing, which is usually spur-gear reduction gearing, may result in accelerated gear wear. Resonant vibrations also may lead to the creation of additional noise as a result of the ever-present tooth backlash and the associated, shock-like tooth contact. In addition, impact stresses in the gears may shorten greatly the useful life of the reduction gearing.
At least for these reasons, reducing engine vibrations not only reduces noise pollution, but also prolongs the service life of the engine. Generally speaking, every effort should be made to avoid spurious resonant vibrations in the operating-speed range of the engine. As a rule, this cannot be done if the propeller and the crankshaft are coupled rigidly, either with or without intermediate reduction gearing.
One solution presented by the prior art is called the “Sarazin pendulum.” While it is true that so-called Sarazin pendulums, which are arranged on the crankshaft webs, can suppress or reduce resonance and vibrations in the crankshaft, they cannot (as a rule) eliminate the first fundamental frequency of the overall oscillating system, which is essentially determined by the propeller's moment of inertia. In addition, crankshafts that incorporate Sarazin pendulums are costly to make and (because of certain design features) are prone to failure. For this reason, the use of such devices is avoided as much as possible, at least in the design of aircraft engines.
It is also known in the prior art to arrange a torsionally-soft coupling between the crankshaft of the reciprocating engine and the propeller. The result of this arrangement is that the first natural frequency of the torsion-oscillating system is substantially lowered. Preferably, the first natural frequency is made lower than the idling speed of the reciprocating engine.
In 1985, in furtherance of this particular prior art solution, Porsche proposed to relieve the load on the crankshaft and the reduction gearing by installing a flexible rubber coupling (that incorporated textile inserts) between the crankshaft and the intermediate gearing. See MTZ Motortechnische Zeitschrift [MTZ Motor Magazine], No. 46 (1985). The flexible rubber coupling was designed to prevent torsional vibration resonance across the entire range of operating speeds. It was also designed to smooth out torsional pulsations out in the drive line between the crankshaft and the propeller shaft.
Despite the advantages offered by this solution, it has been shown that, because of internal friction, a flexible rubber coupling of the kind proposed by Porsche is prone to a comparatively large amount of wear during operation. For this reason, among others, the flexible rubber coupling must be replaced at frequent intervals in order to ensure sufficient operating reliability of the drive unit. In addition, it has been discovered that lowering the natural frequency of the oscillating system may give rise to spurious resonance phenomena when the engine is started, unless additional remedial measures are adopted.
In 1979, Teledyne Industries, Inc. proposed another design for a drive unit to reduce noise generation and component wear. The Teledyne solution was to provide two drive lines between the crankshaft and the propeller. See Overhaul Manual for TIARA Aircraft Engine, Teledyne Industries, Inc., 1979. Each of the two drive lines were provided with different natural frequencies. The first line had a torsionally soft drive line with a low natural frequency. The soft drive line comprised a torsionally soft torsion bar between the crankshaft and the propeller. The second had a torsionally stiff drive line with a higher natural frequency. The stiff drive line comprised a hydraulic coupling.
The torsionally stiff drive line bypassed the torsion bar referred to above and could be partially activated. In the lower speed range (particularly at engine start-up), the torsionally stiff drive line was active, which is to say the torsionally soft torsion bar was bypassed, so that the drive line was operated below its natural frequency (fundamental oscillation). As engine speed increased, the hydraulic coupling was released and the torsion bar was activated, so that the torsionally soft drive line was operated above its resonant frequency.
The Teledyne design ensured that the drive unit never resonated and that unacceptably high stresses in the drive line were avoided. One disadvantage, however, was the fact that two drive lines were required, resulting in a more complex engine. In addition, a relatively costly coupling had to be provided. This entailed not only higher product costs, but also additional costs for controlling the coupling. Furthermore, each coupling was a part subjected to wear. The durability of the components had a significant influence on the service life of the overall drive unit and, thus, had a direct impact on the associated maintenance costs of the engine.
In summary, while the prior art has suggested adequate solutions to the problems identified, the prior art has failed to provide a simple, cost-effective solution to the problem of vibration and noise generation by an aircraft engine.